1. Field of the Invention
This invention relates to oxidic aluminum/zinc catalysts, to processes for their production and to their use as catalysts for the production of unsaturated fatty alcohols.
2. Description of the Related Art
Unsaturated fatty alcohols, i.e. predominantly linear monohydric primary alcohols containing 8 to 22 carbon atoms and 1, 2 or 3 double bonds, are inter alia important raw materials for the production of cosmetic products, such as for example bath oils, make-up creams or intensive hair tonic emulsions [Seifen-xc3x96le-Fette-Wachse, 109, 225 (1983)].
Unsaturated fatty alcohols are generally produced from native fats and oils with high iodine values, for example rapeseed oil or sunflower oil. The natural triglycerides are converted into mixtures of saturated and unsaturated fatty acid methyl esters of corresponding chain length either by pressure hydrolysis and subsequent esterification or by direct transesterification with methanol and are subsequently subjected to catalytic high-pressure hydrogenation at 200 to 350xc2x0 C./250 to 300 bar. The reaction has to be controlled in such a way that only the ester group and not the double bond is hydrogenated. In practice, this high selectivity is normally achieved by using modified Adkins catalysts, for example copper-chromium or zinc-chromium spinels doped with barium or cadmium [W. Keim, Grundlagen der industriellen Chemie, Verlag Salle+Sauerlxc3xa4nder, 1986, page 250]. To produce these spinels, copper oxide or zinc oxide is reacted with chromic acid. Since the handling of chromium(VI) compounds imposes stringent demands on work safety and waste disposal, there is a need for catalysts which, for the same selectivity, are less problematical in regard to their production and use.
Another problem is the fact that, where fats containing significant percentages of linoleic and/or linolenic acid are used, typical hydrogenation catalysts isomerize the polyunsaturated, but isolated double bonds so that increased amounts of conjugene compounds are formed in the resulting fatty alcohols and, in the same way as hydrocarbons which also accumulate as secondary products, can adversely affect the performance properties of the hydrogenation products and are therefore undesirable. Accordingly, another problem addressed by the present invention was to provide hydrogenation catalysts which would be free from these disadvantages.
The present invention relates to oxidic catalysts for the production of unsaturated fatty alcohols containing 8 to 22 carbon atoms by hydrogenation of unsaturated fatty acids, lower fatty acid alkyl esters or unsaturated fatty acid glycerides containing 20 to 25% by weight and preferably 21 to 24% by weight of aluminum and 40 to 50% by weight and preferably 45 to 49% by weight of zinc.
The invention is based on the observation that the oxidic aluminum/zinc catalysts are largely safe both in ecological and in toxicological terms. In addition, it has surprisingly been found that the catalysts according to the invention not only show high activity, they also catalyze the hydrogenation of unsaturated starting materials, for example fatty acids, fatty acid esters or triglycerides, in such a way that the double bonds in the resulting alcohols remain almost completely intact and isomerization to form conjugenes is largely suppressed. In addition, the hydrogenation products are distinguished by an advantageously low hydrocarbon content. Even after storage, the resulting unsaturated fatty alcohols are unusually light-colored and leave only a small residue after fractionation.
Another advantage of the new oxidic aluminum/zinc catalysts is that they can be used in the form of granules so that they do not have to be pelleted before hydrogenation.
Suspension Process
The present invention also relates to a process for the production of oxidic aluminum/zinc catalysts, in which
a) an aluminum salt and a zinc salt in a molar ratio of 5:1 to 1:5, based on the metals zinc and aluminum, are suspended in water,
b) the suspension is thoroughly mixed at elevated temperature and
c) the solid formed is dried, calcined, optionally size-reduced and pelleted.
To produce the catalysts by the suspension process, the aluminum and zinc salts are suspended in water. The solids content of the resulting suspensions can be from 30 to 70% by weight and is preferably from 40 to 60% by weight, based on the suspension. In principle, the thorough mixing of the components may be carried out in any stirring machine. It has proved to be of particular advantage to carry out the thorough mixing of the components discontinuously in a kneader or continuously, for example in an extruder. The thorough mixing of the components may be carried out at elevated temperatures in the range from 50 to 100xc2x0 C. and preferably in the range from 80 to 90xc2x0 C.
Precipitation Process
The present invention also relates to a process for the production of oxidic aluminum/zinc catalysts in which
a) a zinc salt solution is added to aqueous solutions of aluminum salts and alkali metal compounds which have a pH value of 10 to 14,
b) the reaction mixture is adjusted to a pH value of 6 to 10 by addition of acids,
c) the zinc aluminate precipitate formed is separated, washed, dried, calcined and mechanically size-reduced.
The aluminum and zinc salts may be used in a molar ratio of 1:5 to 5:1. In the interests of particularly high activity, long useful lives and low hydrocarbon contents in the hydrogenation products, it has proved to be of advantage to use the aluminum and zinc salts in a molar ratio of 2:1 to 1:2 and, more particularly, in a molar ratio of 1.5:1 to 1:1.5, based on the metals aluminum and zinc.
To produce the catalysts by the precipitation process, an aqueous solution of an aluminum salt and an alkali metal compound, for example an alkali metal hydroxide or carbonate, is initially prepared. To obtain a clear solution, the pH value of the mixture must be in the range from 10 to 14. An aqueous solution of a zinc salt is then added in portions to the highly basic aluminate solution, a precipitate of basic zinc aluminate being formed. It does not matter whether the basic aluminate solution is initially introduced and the zinc salt solution is added thereafter or vice versa. In order quantitatively to precipitate the aluminum/zinc salt, it is advisable to carry out the precipitation with stirring at a temperature of 15 to 60xc2x0 C. and to adjust the pH to a value of 6 to 10 and advantageously to a value of 7 to 8 by addition of acids, for example acetic acid or hydrochloric acid.
Starting Materials
Suitable starting materials for the production of the catalysts according to the invention are the water-soluble, optionally basic carbonates, sulfates, halides, nitrates, formates and/or acetates of aluminum and zinc. The concentration of the salts in water may be from 10 to 70% by weight and is preferably from 30 to 60% by weight, based on the solution. So far as the choice of the zinc and aluminum salts is concerned, the anions may be the same or different. Basic aluminum acetate and zinc acetate are preferably used by virtue of their high solubility in water.
Calcination
After precipitation, the precipitate is filtered off, washed and dried. To this end, the water-containing solid may be spread out in known manner, for example on trays, and dried at 100 to 180xc2x0 C. to constant weight, i.e. to water contents below 1% by weight, based on the dried catalyst. For conversion into the required oxidic form, the dried catalysts are subjected to calcination, i.e. to a solid-phase reaction, at temperatures of 400 to 1,000xc2x0 C. and preferably at temperatures of 500 to 950xc2x0 C. Calcination may be carried out over a period of 1 to 10 hours and preferably over a period of 5 to 8 hours in a stream of hydrogen or in a stream of air. The catalysts are then mechanically size-reduced, for example by granulation.
The oxidic aluminum/zinc catalysts according to the invention are suitable in this form for the hydrogenation of unsaturated fatty acids containing 16 to 22 carbon atoms and 1, 2 or 3 double bonds, esters of these fatty acids with C1-4 alcohols or unsaturated fatty acid glycerides, i.e. mono-, di- and/or triglycerides of aliphatic C8-22 carboxylic acids, which have iodine values of 55 to 125 and, more particularly, 85 to 105.
Hydrogenation is normally carried out at temperatures of 200 to 350xc2x0 C. and under pressures of 250 to 300 bar. The reaction parameters to be established in each individual case are primarily determined by the length of the carbon chains of the esters to be reduced. Within the limits mentioned, long-chain starting products also require high reaction temperatures. In addition, high temperatures also promote reduction of the acid value or saponification value of the hydrogenation product and hence an increase in the yield of fatty alcohols. To modify the process conditions, hydrogenation of the esters may also be carried out in the presence of lower alcohols, low-boiling paraffins or steam.